The present invention relates to thin film bulk acoustic wave resonators. More particularly, the present invention relates to bulk acoustic wave filters and bulk acoustic wave duplexers fabricated from thin film bulk acoustic wave resonators.
As shown in FIG. 1, a thin film bulk acoustic wave (BAW) resonator 10 includes a resonator section 11 based on a layer of piezoelectric material, such as ZnO or AlN , and some include an acoustic mirror 12 (where others, not shown, called bridge-type BAW resonators, include a membrane), all mounted on a substrate 14 made for example from glass. A BAW resonator converts sound waves to electric signals, and vice versa, and can be used as a filter in electronic circuits because of its frequency dependent electrical impedance.
Typically, the acoustic mirror of an acoustic-mirror type of BAW resonator is formed from a combination of layers of materials of differing acoustic impedance. An acoustic mirror is built up on a substrate of for example glass by depositing its various layers of different materials so as to form a stack of layers of different materials on the substrate. Next, a bottom electrode is deposited on the acoustic mirror, and the piezoelectric material is then deposited on the bottom electrode forming a so called piezolayer. Finally, a top electrode is deposited on the piezolayer. The combination of top and bottom electrodes and the piezolayer is called the resonator section of the device. The acoustic mirror serves to reflect acoustic waves created by the piezolayer in response to a voltage applied across the electrodes, thereby isolating the substrate from the piezolayer. FIG. 4 shows the cross sections of two acoustic-mirror type BAW resonators fabricated to operate as parts of band pass filters at substantially different frequencies.
As mentioned above, besides BAW resonators including acoustic mirrors, it is known in the art to provide BAW resonators constructed on a membrane, with an air gap separating the resonator section from the substrate.
Both types of BAW resonators are used as components of filters making up duplexers in e.g. a mobile phone. As illustrated in FIG. 2, a mobile phone can include a duplexer 21, which in turn includes a transmitter (TX) filter 22 and a receiver (RX) filter 23. As shown in FIG. 3, such a filter may be a so-called ladder filter 31. A ladder filter, in general, includes at least one so-called L-section, an L-section including a series resonator and a shunt resonator. The filter 31 shown in FIG. 3 is therefore a ladder filter consisting of two L-sections 3233 connected in series, each L-section 3233 in turn including two resonators, a series resonator 32a 33a and a shunt resonator 32b 33b, the resonators of each individual L-section 3233 being tuned to slightly different frequencies by fabricating one or more layers of the pair so as to have a slightly different thickness (Alternatively, of course, a TX filter 22 or a RX filter 23 of a duplexer 21 may consist of only a single combination 32 of a series resonator 32a and shunt resonator 32b, i.e. a single-stage ladder filter.)
A ladder filter, sometimes called an impedance element filter (IEF), generally consists of one or more so-called L-sections or L-segments, each L-section in turn including one series and one parallel resonator, and thus consisting of an even number of resonators. In some applications, however, a filter consists of an odd number of resonators. For example, a 2xc2xd -stage filter could have either two series resonators and three shunt resonators, or three series resonators and two shunt resonators. The present invention is not restricted to filters having an event number of resonators.
Besides a duplexer 21, a mobile phone can also include other filters (24 of FIG. 2) close in frequency to the duplexer frequencies.
The use of BAW resonators as components of filters in duplexers (then called FBAR duplexers) as taught by the art involves using two separate substrates for each filter, with external passive surface mounted devices (SMDs) as components.
What is needed is a way of fabricating on a single substrate the two filters for use in an FBAR duplexer. Ideally, other filters included as a component of the equipment using the duplexer (such as a mobile phone) and operating at a frequency near the frequencies of the duplexer could then also be advantageously fabricated on the same substrate. Such a fabricating would reduce the size of the equipment owing to the duplexer and filters operating near the frequencies of the duplexer, and would also, in principle, reduce the cost of fabricating the equipment.
Accordingly, the present invention provides, a monolithic bulk acoustic wave (BAW) duplexer, and a method for fabricating same, the duplexer having a transmitter section as a first component filter and a receiver section as a second component filter, both component filters fabricated on a single substrate and both including at least one shunt BAW resonator and one series BAW resonator, each BAW resonator including a resonator section atop an isolation structure provided so as to separate the resonator section from the substrate, including: a patterned bottom electrode material for use as the bottom electrode of each of the resonators of the duplexer; a patterned piezoelectric material for use as the piezolayer of each of the resonators of the duplexer; a patterned top electrode material for use as the top electrode of each of the resonators of the duplexer; a tuning layer for the shunt resonator of each of the two component duplexer filters; and a tuning layer for both the series and shunt resonators of one of the two component duplexer filters.
In a further aspect of the invention, the shunt tuning layer is provided in a location in each shunt resonator that is either: between the mirror and the bottom electrode; between the bottom electrode and the piezolayer; between the top electrode and the piezolayer; or on top of the top electrode.
In another, further aspect of the invention, the tuning layer for both the series and shunt resonators of one of the two component filters is provided in a location, in both the series and shunt resonators of the component filter, that is either: between the mirror and the bottom electrode; between the bottom electrode and the piezolayer; between the top electrode and the piezolayer; or on top of the top electrode.
In some applications, each isolation structure is an acoustic mirror.
In yet still even another aspect of the invention, the duplexer also includes at least one planar spiral inductor provided in the course of depositing one or another layer of material in building up the duplexer, the planar spiral inductor having coils spiraling outward substantially in a plane from an innermost coil to an outermost coil.